This invention relates to a process for the surface treatment of articles fabricated of austenitic iron-nickel-chromium based alloys, to resist and to deter the onset of intergranular cracking and corrosion. The process comprises at least one cycle of cold deformation of the near surface region, for example by high intensity shot peening, followed by recrystallization heat treatment. The novel process can be applied to wrought, cast or welded materials, and is particularly suited for in-situ or field application to components such as steam generator tubes or core reactor head penetrations of nuclear power plants.
The prior art presents examples of the use of surface cold work, for example by xe2x80x9cshot peeningxe2x80x9d, to effect a state of residual compression at the surface of a material, and thus render the material resistant to the initiation of cracks which require a tensile stress for initiation and propagation. Shot peening is a method of cold working, inducing compressive stresses on and near the surface layer of metallic parts. The process consists of impinging on the test article a stream of shot, directed at the metal surface at high velocity under controlled conditions.
Although peening cleans the metal surface, the major purpose is to impact and enhance fatigue strength. Peening processes are known to relieve tensile stresses that contribute to stress-corrosion cracking. Yamada in U.S. Pat. No. 5,816,088 (1998) describes a surface treatment method for a steel workpiece using high speed shot peening. Mannava in U.S. Pat. No. 5,932,120 (1999) describes a laser shock peening apparatus using a low energy laser. Harman and Lambert in U.S. Pat. No. 4,481,802 (1984) describe a method of peening the inside of a small diameter tube in order to relieve residual tensile stresses.
Friske and Page in U.S. Pat. No. 3,844,846 (1974) describe a surface deformation treatment by shot peening, which is applied to austenitic Crxe2x80x94Fexe2x80x94Ni alloys without subsequent heat treatment, in order to render the surface region highly deformed, and subsequently more resistant to intergranular corrosion in the event that the article becomes exposed to sensitization temperatures, i.e., 400-700xc2x0 C., during service.
Kinoshita and Masamune in U.S. Pat. No. 4,086,104 (1978) also describe a surface deformation treatment for austenitic stainless steel components, applied following final mill annealing or hot rolling treatments, which renders the surface of the stainless steel more resistant to oxide scale formation during subsequent exposure to high temperature steam.
Anello in U.S. Pat. No. 4,495,002 (1985) describes a three step process for martensitic stainless steels to increase their resistance to chloride corrosion, wherein, an article is subjected to surface deformation via shot peening, followed by an aging treatment at 527-549xc2x0 C., and followed by a final lower intensity shot peening. In such manner, a homogeneous near surface region consisting of aged martensite is obtained which is resistant to chloride corrosion and cracking.
Palumbo in U.S. Pat. No. 5,702,543 (1997) and U.S. Pat. No. 5,817,193 (1998), describes thermomechanical mill processes involving the application of bulk cold work, such as cold drawing or cold rolling, followed by a recrystallization heat treatment to improve the grain boundary microstructure of austenitic Nixe2x80x94Fexe2x80x94Cr alloys and thereby effect significant improvements in intergranular corrosion- and cracking-resistance.
We have discovered that finished and semi-finished articles made of austenitic Nixe2x80x94Fexe2x80x94Cr alloys, whether in the wrought, forged, cast or welded condition, may be subjected to cold deformation of the near surface region by a technique such as shot peening, followed by annealing of the article at a temperature below its melting point for a time sufficient to induce recrystallization in the cold-worked near surface region. In this specification, xe2x80x9cnear surface regionxe2x80x9d refers to the surface layer of the article to a depth in the range from 0.01 mm to about 0.5 mm.
It is a principal object of this invention to provide a surface treatment methodology which will alter the recrystallized structure in the near surface region of a finished article or component made of austenitic Nixe2x80x94Fexe2x80x94Cr alloys to impart significant resistance to intergranular corrosion and cracking during service of the article or component, without the need for bulk deformation thereof by a process of rolling, extruding, forging or the like.
It is a further object of this invention to provide a surface treatment process as aforesaid, which may be used to treat and improve the degradation and corrosion resistance of finished parts of complex shape and parts which may already be in service, in particular, nuclear steam generator tubes, nuclear reactor head penetrations and the like.
With a view to achieving these objects, there is provided a method for improving intergranular corrosion and cracking resistance of an article fabricated of an austenitic Nixe2x80x94Fexe2x80x94Cr alloy by subjecting the alloy to at least one cycle comprising the steps of:
(i) cold working the surface region of the article to a depth in the range of from 0.01 mm to about 0.5 mm; and
(ii) annealing the article at a temperature below the melting point of said alloy for a time sufficient to induce recrystallization in said surface region.